The need for energy has spawned numerous new techniques for recovering hydrocarbons from more and more challenging reservoir environments. One such effort has been the recovery of gas from tight rock formation. In these formations, the rock has a low porosity but the entrapped gas is at high pressures within the rock. Wells are drilled into this rock to liberate the gas and collect it in the well itself for recovery and distribution. Such wells are cased with a steel liner which is cemented to the formation. To generate a communication channel to the rock containing the gas, perforation charges are detonated within the well punching a hole through the steel, cement, and into the rock formation. Penetration of the shaped charge used to penetrate the formation normally extends from a few inches to less than 10 feet. This provides very limited penetration into the gas bearing rock and, therefore, limited ability for the gas to move to the well bore.
To improve communication between the well bore and the rock, hydraulic fracturing of the rock is performed. High pressure water is pumped into the well, through the perforation in the casing and cement, and into the damaged formation caused by the perforation charge. As the pressure exceeds the formation pressure, the rock begins to break along preferential weaknesses. This opens fissures in the rock that extend in vertical fractures. That is, the fractures that are created tend to be larger in the vertical direction than in the horizontal direction by several orders of magnitude. Propagation tends to be along stress lines already in the formation. These fissures then form a pathway for the gas to move from the face of the fractured rock to the well bore for recovery. The high pressure forces the gas out of the minimally porous rock when it experiences a significant drop in pressure. The fracture pressure represents hydraulic pressures on the order of the formation pressure (essentially due to the weight of the layers of rock above). After the relaxation of pressure of the hydraulic fracturing fluid, the pressure in the fracture is minimal in comparison to the rock. This means the rock experiences an relatively low surface pressure while experiencing extremely high internal pressures. Diffusion of gas from the internal rock can then flow to the fissure which essentially creates a channel to the well bore. The lack of porosity in the rock eventually depletes the gas at the surface of the rock which exposes the interior rock to the reduced pressures. As this progresses, the depletion layer moves in ever more slowly. This means that for a larger recovery of gas from the rock the greatest value is from the greatest exposure of the surface area of the rock rather than simply providing a pathway for the gas to pass to the well.
Fracturing the rock both provides a greater surface area for diffusion from the rock and improves diffusion of the gas through the rock from the high pressure zone. The greater the surface area exposed, the greater the initial gas produced. However, this also facilitates long term recovery of gas. Therefore, it is beneficial to maintain the transport properties of the fracture.
Hydraulic fracturing causes the rock to separate to create the fracture channel. As the rock fractures, the parted rock is composed of a surface with an opposing mating surface. If the hydraulic pressure is reduced, the result is that the two rock surfaces come together, effectively mating. Thus no channel remains.
To ensure a continuing channel, proppant is inserted into the fracture. The expressed purpose of the proppant is to prop open the channel. This prevents the closure of the rock faces, negative and positive, created upon fracturing due to the randomness of the interstitial proppant material.
While it is useful to create a separation of the two surfaces of the rock composing the wall of the fracture, proppant would be of little value if it did not provide a highly conductive zone to the well bore from the rock face. Granularity produces this conductive character by creating interstitial spaces between the particles composing the proppant through which the gas can flow. The proppant must, then, be strong enough to hold open the formation without losing this interstitial space or percolation network. Failure of the proppant can occur if the formation pressure is so great that the stress on the particle exceeds its strength and the proppant particles break. When this happens, the particles are forced closer together reducing the overall void space comprising the percolation network. In addition, structural failure can produce fragments of the proppant which further fill the void space and reduce overall conductivity.
There arises the challenge of finding a material which can withstand the closure pressures without losing its percolation network while facilitating flow into the formation crevices to maximize the amount of the fracture that remains open.
Materials which have been used for this purpose have been, for the most part, naturally occurring. Probably the most common is various types of sand. The irregular shapes of the particles prevent close packing resulting in a natural percolation network. The structural strength of the sand prevents formation closure and maintains the network. However, two primary shortcomings are associated with sand. First, the specific gravity of sand tends to be approximately 2.8 making it significantly heavier than the water, a specific gravity of 1.0, or brine, a specific gravity of as much as 1.2, which is used for fracturing and to carry the proppant into position. As a result, the sand tends to settle out during the insertion process. To avoid this, additives are mixed into the water or brine to increase the viscosity and, in some cases, the specific gravity to extend the settling time of the sand.
While this improves the depth to which the proppant penetrates into the fracture, it is believed that little of the proppant reaches more than a relatively small percentage of the total fracture zone. Additionally, the thickening agents themselves have deleterious effects. Under high pressure, the agents can be pressed into the formation rock further reducing its porosity and permeability. The agents and fracturing fluids flow back up the well bore causing a disposal problem. Additionally, some fraction of these agents typically remains trapped in the proppant bed, reducing its permeability.
To optimize the flow of proppant into the fracture while eliminating or reducing the need for thickening agents, proppant must match as closely as possible the specific gravity of the hydraulic fracturing fluid. This would keep the proppant material suspended in the fracturing fluid to allow it to penetrate into all segments of the formation into which the fluid penetrates.
Several techniques are used to reduce the specific gravity of proppant. One is to select a proppant which has a lower specific gravity. Examples of just such a low specific gravity are organic materials such as walnut shells, pits, husks, and the like. However, these lighter materials tend to introduce other limitations. While walnut shells would penetrate deeper into the formation, their structural strength limits their applicability to relatively small formation closure pressures. Crushing the walnut shells eliminates the spaces between the shell fragments, fills them with newly produced small broken particles, and drastically reduces the percolation network.
Another method of reducing the specific gravity of proppant has been to produce light materials as the core of the proppant, most often ceramics and metal oxides, which are then bound together into a particle by use of an adherent or through sintering. Kaolin, clays, and alumina are often used as precursors which may be bound together with a sacrificial binder which is burned off or becomes part of the chemical processing during sintering. Glass spheres both naturally occurring as well as manmade may also be incorporated. This composition provides improved structural integrity and reduced density. However, the material may remain porous and allow degradation during exposure to the fracturing fluid or well flow. And, the particle strength must be balanced against the degree of structural integrity desired. The greater the structural integrity, the greater the general density of the particle, and the less the buoyancy.
Methods of making organic and ceramic proppants less porous and structurally stronger include various methods of coating them with sealing or hardening shells. This does improve their strength but with limitations and at the cost of some of the buoyancy. The harder shell tends to increase the density overall of the proppant particle which requires the core to be lighter and, more likely, less structurally sound. It becomes a balancing effort to add coating at the least cost to proppant weight while increasing the overall strength.
Another method of reducing the specific gravity of proppant while retaining structural strength is to coat a strong but dense proppant with a low specific gravity material such as microspheres. The aggregate specific gravity of the coated particle then is reduced. Coated particles can be better suspended in the fracturing fluid. Once in place, the formation closes on the coated particle causing the coating to break off the particle or, if soft to deform. This may reduce the void space and, therefore, the percolation network.
While coating proppants may reduce their specific gravity, the fundamental properties of the proppant are generally not changed. While some coatings are meant to harden the exterior of the proppant thereby contributing to some strength, coated proppant ultimately behaves as the base particles. Sand, if coated, can have its specific gravity reduced. However, the material strength of the sand remains limited and, if exceeded, fractures producing small fragments which can occlude the interstitial spacing comprising the percolation network. Some coatings may be applied to capture these fragments but then are not designed for reduced or neutral buoyancy. While the coating may reduce the flow of small fragments, the overall change of the size of the fragments of a particle once broken by entropy will occupy more space than they did before breakage. In addition, the coating will occupy space. Therefore, even coated proppant meant to capture these “fines” will have reduced conductivity if the particle fails.
Recognizing the need for stronger proppants, especially for deeper wells where the formation closure pressures are greater and that many of the more common proppants such as sand will fail, stronger proppant which tend to be manufactured have been developed. Ceramic proppants are a primary class of just such a manufactured material. These materials provide strength that allows the proppant to withstand formation closure pressures at depths in excess of those at which sand and other more common proppants fail. However, the materials necessary to produce these hard materials provide strength but at a significant cost in specific gravity. Many manufactured proppants have specific gravities as high as 3.8. While the hardness reduces the structural shortcomings, it exacerbates the difficulty of placement into the fractures.
The primary method of countering excessive specific gravities of proppants has been to thicken the fracturing fluids with various polymerizers. The gel-like consistency allows the heavier proppants to be flowed deeper into the fractures by extending the settling time. Pressures are then increased and higher pumping speeds used to move the fluid and proppant into the fracture. However, this same pressure forces some of the fracturing fluid into the pores of the formation rock. As a result, the already low permeability rock has its pores filled with the thickening agents. It is believed that this further reduces the recovery of gas or oil from the reservoir. It would then be especially beneficial if the amount of thickening agents necessary were either reduced or eliminated all together.